A number of advanced coating systems are often applied to one or more surfaces of metallic components or metallic portions of components formed from other materials. The coatings are often used to protect the surface or substrate, providing electrical or thermal insulation, or providing wear resistance, corrosion resistance, or oxidation resistance. A wide variety of specialty materials are available for these uses, including thermal barrier materials (e.g., stabilized zirconia) that are used to protect superalloy parts and other high-temperature articles. Other protective coatings can be formed from many different alumina, carbide, boride, or silicide compositions.
Other specific examples of specialized compositions include various aluminum-silicon coatings, which can include aluminum-silicate materials, aluminide silicide materials, and the like. In some instances, the various coatings mentioned above are used for purposes other than surface protection. For example, they may be used for cosmetic purposes, or as gas-barrier coatings; or for masking sections of a substrate while other processing steps are carried out on the substrate.
Many of the materials described above are designed to be permanently deposited on the substrate, and not removed for the life of the part or device. As an example, some of the aluminum silicate materials are applied to a surface in slurry form. The coatings can be heat-treated to remove any volatile materials such as organic binders, or sometimes simply air-dried at ambient temperature. Usually, the coatings are then fired onto the substrate surface, e.g., at temperatures above 500° C. and sometimes above 1,000° C., to permanently bond the coating to the substrate. The highly adherent coatings of this type can protect a variety of articles from the severe effects of temperature, corrosion, abrasion, and the like.
However, some of the performance coatings described above are very useful for applications in which the coatings need to be removed at some point. One example relates to various coatings used as masking materials. As described in a pending U.S. patent application Ser. No. 14/068,937, methods for preparing magnetic components require the masking of pre-selected sections of the component surfaces. After the components are heat-treated and nitrided in a specialized reaction, the masking material or “stop-off” needs to be removed.
However, conventional materials and processes for removing various masking materials from metallic substrates exhibit drawbacks when used for aluminum-silicon coatings like the aluminum silicates. Some of the coating removal materials are unable to remove most of the masking material from the surface. Other coating removal materials may be somewhat effective in removing the masking material, but damage or otherwise modify the underlying substrate.
With these observations and concerns in mind, improvements in methods and compositions for removing aluminum-silicon coatings from a metallic substrate would be welcome in the art. The new technology should improve the ability to remove the coatings, e.g., masking materials, without adversely affecting the substrate. The materials and processes should also be relatively economical to obtain and implement. Moreover, use of the new materials should not involve difficult issues regarding safety or health procedures.